Thermistor circuit



Oct. 6, 1942. 1 H BOLLMAN 2,298,192

THERMISTOR CIRCUIT Filed DGO. 25, 1941 trolling degree by thev ielf.

f less value between tential to a harmless l circuit diagram of 22, i939, modiiied I reedbaek pliiier is shunted across the feedback path, the value of UNITED" s'rAT THEBMISTOR CIRCUIT John n. Bouman, Rutherford, N.

Bell Telephone York, N. Y.,

J., assigner to Laboratories, Incorporated, New a corporation of New York Application December 26, 1941, Serial No. 424,455 8 Claims. (C1. 219-19) 'The present invention relates to circuits employing resistors of high temperature coeiilcient of resistance, commonly called therxnistors.`

4More specifically, the invention relates tocirvcuitsiusing thermistors equipped wlthheaters o A or controlling their "temperature in order to r -control their resistance. v'@ferred `toas indirectly heated-the`rmistors in These are often recontradistincti'on to 'directly heated l (orf selfheated) thermistors .which are heated in icon- RIz loss 'the thermistor 'Ihe heating coil is usually mountedzwith x-close thermal coupling to the thermistor-element.

In one type, the thermistor is a bead or block of some suitable substance having high temperature coeiiicient of resistance placed inside a glass' tube and the heating coil is wound on the'outside ofthe glass tube. Y. It is found that in cases where there is a large dinerenoe of direct current potential between the thermistor elementg'and heater winding, deterioration of the thermistor is liable to occur in time because of electrolysisl through the glass or other insulation separating the` thermistor and heater. 'I'hermistors are usually semiconductors consisting of oxides, sulphides or other compounds subject to disintegration by electrolytic action and itis found that this action can occur through very high resistance insulation if a steady differenceof oi time,

An obiect of the present invention is to protect against deterioration of this type by effectively reducing the diilerence of potential to a harmthe thermistor element and its heater winding.

'I'his obiect is achieved in accordance with the invention by providing a leak p ath for direct current between the thermistor element and heat coil sumcient to reduce the steady drop of povalue.

Reference will now be made to the accompany- Aing drawing for a detailed description 'of one manner in which the invention may be practiced. The single figure of the drawing is a schematic ai multiplex carrier repeater type .disclosed l in United lStates Anderson- `'et al.-I 2,170,046, August to incorporate the present inoi' the general patent to lIi'. B.

potential is applied over a long period .gain comprises a thermistor 'is controlled by .feedback conductor 9 the impedance being controlled by a pilot wire regulator.

The present invention is embodied, Ain the present disclosure, in a similar amplifier in which the variable impedance for controlling the whose temperature the pilot channel regulating circuit.

The amplifier itself is shown as comprising three stages I, 2 vand 3 with the input line or circuit at 4 coupled to the grid of stage i and the output line or circuit at 5 coupled to the plate of stage 3. 'I'he output stage includes in its output a bridge 6 lwith conjugate branches leading respectively to output transformer 'I and feedback potentiometer l. 'I'he details of this bridge and of the rest of the ampliiler circuit including interstages and other circuit elements have not been illustrated since they form no part of the present invention and they may bevas disclosed, for example, in the Anderson et al. patent.

The feedback connection leads from an adjustable tap on potentiometer 8 via conductor 0 and stopping condenser I I) to the grid of stage I. The cathodes are grounded through individual cathode networks Il, I2 and I 3, For purposes of gain control- (in addition to potentiometer l) the thermistor I4 is connected between blocking condenser I5. (It is desirable to place condenser I5 on the ground side of the thermistor.) Impedance elements maybe associated with the thermistor to modify its action as desired, such being indicated at I6 for example. Plate voltage is supplied to the plate of stage I from battery I1 having its positive pole suitably connected to a point in the output bridge, as in the Anderson et al. disclosure.

'I'he impedance of the thermistor shunt is controlled by supplying variable amounts of current to the heater winding 20 which has thermal coupling with the bead or element I4 but i's insulated electrically therefrom, by a thinwall of glass or other suitable insulation. 'I'he insulation resistance may amount to the order of 100v mcgohms. The heating current might be con- I trolled in any suitable manner, even by hand.

vention. -`In the Ydisclosure ci" that patent a' three-stage ampliiieris provided with a negative connection'and the gain of the amcontrolled'by a variable impedance 2,246,307, June 17, 1941.-

but in a system such trolled from a pilot channel regulating circuit indicated at 2|. This may be of the type shown. for example, in United States patent to Krist It includes an input.

circuit 22 for deriving the pilot current froma suitable point in the circuit s, together with nitering, amplifying and rectiiying circuits for de and ground in series withv as that illustrated it is ecn'- j 2 veloping a heating current and supplying it through output transformer 23 to the circuit 24 leading to heater 20. The heating current r'nay be 60-cycle alternating current of varying effective value determined by the pilot regulating circuit.

It will be noted that they thermistor I4 is at substantially the direct potential of the plate of the output stage, such as 130 volts, for example. Circuit 24 is shown having no direct conductive path to ground but it has unavoidable iinite resistance to ground represented by the insulation resistance of transformer 23 and other associated circuit elements. This insulation resistance for direct current may be assumed to amount to several megohms. This and the insulation resistance of the thermistor glass insulation are found suillciently low in practice to permit polarization of the glass to take place and a minute leakage current to now across from the thermistor I4 to the heater sufficient to cause deterioration in the thermistor after a suflicient period of service.

In accordance with the invention, this deterioration is prevented by connecting a leak resistor of, for example, 1 megohm resistance between the thermistor I4 and heater 20. This resistance is low enough in comparison with the insulation resistance of the thermistor glass insulation and circuit 24 to ground so that the heater 2IJ is brought to nearly the same direct current potential as the thermistor I4 and leakage current through the insulating wall between I4 and 20 is substantially prevented. The resistance of 25 may be varied within rather wide limits and may be, instead of 1 megohm, forvexample, 100,000 ohms in the circuit illustrated.

The thermistor I4 with its heating winding 20 may, for illustration, be constructed as disclosed in United States patent application of G. L. Pearson Serial No. 286,307, filed July 25, 1939.

The invention is not limited to use with a feedback amplier or with any type of amplifier but it is capable of use wherever a steady dilerence of potential is to be reduced to innocuous value between the thermistor element and heater.

What is claimed is:

1. An indirectly heated type thermistor, a heater therefor, means tending to maintain the thermistor at an unidirectional potential diierent from that of the heater and a leak resistance connected between the thermistor and the heater to reduce the vpotential difference to negligibly low value.

2. An indirectly heated thermistor, a heater therefor, means tending to maintain the thermistor resistance at unidirectional potential above that of the heater an amount suicient to deleteriously aiect the thermistor resistance, and means for raising the steady potential of the heater with respect to that of the thermistor resistance sufficiently to materially reduce the deleterious effect of steady potential difference between the thermistor resistance and the heater comprising a resistance connected between the thermistor element and the heater having a magnitude small compared to the minimum operating magnitude of the insulation resistance between the thermistor and the heater.

3. An indirectly heated thermistor, a heating element therefor, means maintaining the thermistor at unidirectional potential above ground,

and a resistance connected between the therthe thermistor and the heatingelment, the insulation resistance between the heating'element and ground having an order of magnitude at least as large as said resistance connected between the thermistor and the heating element.

4. An indirectly heated thermistor, a heating element therefor, means maintaining the thermistor at'unidirectional potential above ground, and a resistance connected between the thermistor and the heating element having a magnitude small compared to the minimum operating magnitude of the insulation resistance between the thermistor and the heating element and large compared to the magnitude of the resistance interposed by said means between said thermistor and ground.

5. An amplifier comprising a space discharge device, a space current supply source for said device, an indirectly heated type thermistor, a heating element therefor, a negative feedback path for said amplifier including the thermistor, means for electrically insulating the thermistor heating element from ground, means including a portion of said path conductively connecting a point of positive potential on said source to the thermistor resista-nce and maintaining the thermistor resistance at unidirectional potential above ground,

and a resistance of the order of a megohm connected between the thermistor and the heating element.

6. A thermistor comprising a thermistor resistance having a large temperature coeilicient of resistance, a heating element therefor, solid heat conducting electrical insulating material interposed between and in contact with said resistance and said heating element, and a resistance of the order of a megohm connecting said thermistor resistance and said heating element.

7. The combination with an indirectly heated thermistor having heat conducting solid electrical insulation between and in contact with the thermistor resistance and the heating element, of means maintaining the thermistor resistance at an unidirectional potential above ground, and means for maintaining the heating-element at substantially the same average potential as the thermistor resistance.

8. The combination with a thermistor device comprising a temperature-dependent resistance and a heating element therefor, of means tending to maintain said temperature-dependent resistance at unidirectional potential sufficiently above that of saidheating element to result in deterioration of said temperature-dependent resistance, and means for maintaining the unidirectional voltage between said temperature-dependent resistance and said heating element at innocuous value comprising a leak resistance connected between said temperature-dependent resistance and said heating element.

JOHN H. BOLLMAN. 

